Strains of Bifidobacterium, such as Bifidobacterium longum (hereinafter referred to as bifidus bacteria), are some of the predominant bacteria of intestinal microflora which may be formed in the intestinal tract of humans, and it is known that consumption of the bacteria is effective for improving the intestinal environment. Recently, demands for food containing bifidus bacteria has increased, and bifidus bacteria have been widely used in cultured milk, acidophilus milk, and so forth. Also, various reports have been made on the bioactivity of the bifidus bacteria, such as antitumor activity, and attempts have been made to utilize the bioactivity for various health foods. As described above, while it is anticipated that bifidus bacteria is effective for, for example, maintenance of the intestinal microflora and improvement thereof, it is necessary to continuously consume the bifidus bacteria to make the bifidus bacteria exert their effect.
However, the bifidus bacteria do not grow well on a milk base medium, and it takes a long period of time for the pH and the number of bacteria to reach a certain value. Also, since it is difficult to preserve the bifidus bacteria under acidic conditions and the bacteria easily die, it is a significant problem to maintain the number of the bacteria during distribution.
On the other hand, although it is said that living bifidus bacteria have the most effective bioactivity, various factors killing or inhibiting the growth of lactic acid bacteria and bifidus bacteria exist in the digestive tracts of humans. In particular, these bacteria must survive low pH of gastric juice and digestive enzymes when it reaches the stomach which is the first hurdle of the digestive tract. Although it depends on the contents, etc., in the stomach, one of the general references of the gastric juice tolerance is to survive for two hours at pH 3.0 (Itoh et al., Nicchiku Kaihou, Vol. 63, No. 12, pp. 1276-1289, (1992)).
However, most of the bifidus bacteria are weak at low pH, and it is said that the probability that one will pass through the gastric juice is lower than for lactobacilli, etc.
Accordingly, it is desired, in terms of developing foods and beverages containing the bifidus bacteria, to develop bifidus bacteria which is capable of growing to a certain number in a short period of time (i.e., fermentation characteristic), maintaining more than a certain number of living bacteria during distribution after its production or during consumption (i.e., acid tolerance), and exerting bioactivity by reaching the digestive tract (intestinal tract, in particular) alive using its gastric acid resistance.
From the viewpoints mentioned above, studies have been made on the fermentation characteristic, acid resistivity, and gastric acid resistivity of the bifidus bacteria. For example, it is known that a variant of Bifidobacterium bifidum exists which has acid tolerance and fermentation characteristics in a milk base medium (Japanese Examined Patent Application, Second Publication No. Sho 56-42250). Also, it is known that a strain of Bifidobacterium longum has acid tolerance (Japanese Examined Patent Application, Second Publication No. Sho 59-53829 (hereinafter referred to as the cited document 1) and Japanese Unexamined Patent Application, First Publication No. Hei 11-75830). Moreover, it is known that Bifidobacterium breve grow in a reconstituted skim milk medium and has acid tolerance (Japanese Unexamined Patent Application, First Publication No. Hei 4-320642).
However, it takes more than fifteen hours for the above-mentioned Bifidobacterium bifidum and Bifidobacterium breve to ferment the milk base medium to a certain pH. Also, the survival rate of many of the acid resistant strains, when stored in an acidic milk product or fruit juice (pH 4.0-4.8) in a refrigerator, is not satisfactory. That is, the acid tolerance of the above-mentioned bacterial strains is not sufficient. Also, although there is a report (Japanese Unexamined Patent Application, First Publication No. Hei 9-322762 (hereinafter referred to as the cited document 2) on a mutant strain of the Bifidobacterium longum having a strong gastric juice tolerance which is said to be related to the survival rate in a digestive tract (intestinal tract), the preservability of the mutant strain is not sufficient, as is also obvious from the results of tests which will be described later.
Accordingly, it is an object of the present invention to provide bifidus bacteria having excellent growth rate in a milk base medium, and acid tolerance, and whose number can be readily maintained to be more than a certain number in a cultured milk product and during circulation. The bacteria also has a strong gastric juice tolerance, and exerts bioactivity by reaching the digestive (intestinal tract) alive.
Also, it is an object of the present invention to provide powders, foods and beverages containing the bifidus bacteria.